netdev - Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side sk_callback

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Message-ID: <ef09820a-ca97-0c50-e2d8-e1344137d473@I-love.SAKURA.ne.jp>
Date:   Mon, 21 Nov 2022 19:03:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Tom Parkin <tparkin@...alix.com>,
        syzbot+703d9e154b3b58277261@...kaller.appspotmail.com,
        syzbot+50680ced9e98a61f7698@...kaller.appspotmail.com,
        syzbot+de987172bb74a381879b@...kaller.appspotmail.com
Subject: Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side
 sk_callback_lock

On 2022/11/21 18:00, Jakub Sitnicki wrote:
>> Is it safe to temporarily set a dummy pointer like below?
>> If it is not safe, what makes assignments done by
>> setup_udp_tunnel_sock() safe?
> 
> Yes, I think so. Great idea. I've used it in v2.

So, you are sure that e.g.

	udp_sk(sk)->gro_receive = cfg->gro_receive;

in setup_udp_tunnel_sock() (where the caller is passing cfg->gro_receive == NULL)
never races with e.g. below check (because the socket might be sending/receiving
in progress since the socket is retrieved via user-specified file descriptor) ?

Then, v2 patch would be OK for fixing this regression. (But I think we still should
avoid retrieving a socket from user-specified file descriptor in order to avoid
lockdep race window.)


struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
                                struct udphdr *uh, struct sock *sk)
{
	(...snipped...)
        if (!sk || !udp_sk(sk)->gro_receive) {
		(...snipped...)
                /* no GRO, be sure flush the current packet */
                goto out;
        }
	(...snipped...)
        pp = call_gro_receive_sk(udp_sk(sk)->gro_receive, sk, head, skb);
out:
        skb_gro_flush_final(skb, pp, flush);
        return pp;
}

> 
> We can check & assign sk_user_data under sk_callback_lock, and then just
> let setup_udp_tunnel_sock overwrite it with the same value, without
> holding the lock.

Given that sk_user_data is RCU-protected on reader-side, don't we need to
wait for RCU grace period after resetting to NULL?

> 
> I still think that it's best to keep the critical section as short as
> possible, though.